1. Field of the Invention
The present invention relates to a light-shielding aluminum nitride sintered body and a method of manufacturing the same and, more particularly, to a light-shielding aluminum nitride sintered body suitably used as a substrate of a circuit board and a method of manufacturing the same.
2. Description of the Related Art
An aluminum nitride (AlN) sintered body has a thermal conductivity much higher than that of alumina, a good insulation property, and a thermal expansion coefficient close to that of silicon (Si). Therefore, it is attempted to apply an AlN sintered body to a substrate of a circuit board for mounting a semiconductor device.
U.S. Pat. No. 4,847,221 discloses an AlN sintered body consisting of an A%N single phase and a second phase containing 0.01 to 8,000 ppm of a rare earth element and 2,000 ppm or less of oxygen and having a thermal conductivity of 200 W/m.multidot.K or more. Since such an AlN sintered body has a high thermal conductivity, an undesirable phenomenon which has not been encountered in conventional sintered bodies is caused. That is, a high thermal conductivity of the above AlN sintered body is achieved by a high purity of the sintered body. An AlN sintered body having a high purity, however, inevitably exhibits transparency as its essential property. The transparency is an undesirable characteristic depending on the field of applications. For example, when the above AlN sintered body is used as a substrate of a circuit board, a semiconductor chip such as an IC chip or an LSI chip mounted on the upper surface of the circuit board may be irradiated with light transmitted through the lower surface of the circuit board to cause an erroneous operation by a photoelectric effect. In addition, since a slight color tone difference is conspicuous in a substrate consisting of a transparent AlN sintered body, its outer appearance is degraded.
In order to solve the above problems, U.S. Pat. No. 4,883,780 discloses an AlN sintered body which is colored dark gray, black, or the like. This AlN sintered body consists of (a) AlN, (b) at least one member selected from the group consisting of a rare earth aluminum compound, an alkali earth compound, and a rare earth alkali earth aluminum compound, and (c) at least one member selected from the group consisting of Hf, Mo, W, and transition metals belonging to the groups Va, VIIa, and VIII of the periodic table. The AlN sintered body contains 0.01 to 18 wt % (oxide content) of the component (b) and 0.01 to 15 wt % of the component (c). Since, however, at least one of a rare earth aluminum compound, an alkali earth compound, and a rare earth alkali earth aluminum compound as the component (b) is present in an AlN grain boundary or the like of such an AlN sintered body, the thermal conductivity of this AlN sintered body is unsatisfactory, i.e., a maximum of at most about 110 W/m.multidot.K.
As described above, it is difficult to obtain an AlN sintered body having both a high thermal conductivity and a satisfactory light-shielding property by the conventional techniques.